DPF Warning Light On? Here Is What To Do

What a DPF is

A diesel particulate filter, or DPF, sits in the exhaust of most modern diesel cars and vans. Its job is to trap the soot that diesel engines produce, so it does not come out of the tailpipe.

Like any filter, it slowly fills up. To clear itself, the car burns the soot off at high temperature. This is called regeneration, and when it works properly you will not even notice it happening.

Why the light comes on

Regeneration needs the exhaust to get very hot, which usually means a steady run at speed. If the car only ever does short trips, such as the school run or popping into Walsall town centre, the exhaust never gets hot enough for long enough and the soot builds up.

When the filter gets too full, the DPF warning light comes on. It can also come on because of a fault elsewhere that stops regeneration from working, such as a faulty sensor, a sticking EGR valve, a problem with the injectors or, on some cars, low additive fluid.

If the car tries to regenerate during a journey and you switch off before it finishes, the process is interrupted. Signs a regeneration is under way can include the cooling fan running after you park, a higher idle speed or a hot smell from the exhaust. If you notice those, it is worth carrying on driving for a few more minutes if you can.

What to do first

Check your handbook, because the advice varies between makes. On many cars, a drive of around 15 to 20 minutes at a steady speed, often 40 to 50mph or more on a dual carriageway or motorway, gives the car the chance to regenerate and the light goes out.

If the light stays on after that drive, or other lights come on with it, such as the engine management light, or the car goes into limp mode with very little power, stop trying to clear it yourself and get it checked. Driving on with a badly blocked filter can put strain on the engine and turbo.

How we deal with it

We start with a diagnostic check to read the fault codes and see how full the filter is. Just as importantly, we look for the reason it blocked in the first place. If we only clear the filter and leave the cause, the light will be back.

Depending on what we find, a filter can often be cleared by a forced regeneration with diagnostic equipment, or by cleaning. Over many years, ash builds up in the filter that regeneration cannot burn off, and at that point the filter may need replacing. We will explain which applies to your car and give you a quote before any work starts.

Why we do not remove DPFs

You may hear of people having their DPF taken out. It is illegal to drive a car on the road with its DPF removed if it was fitted from the factory, and it is an MOT failure. It can also affect your insurance.

We will only ever repair, clean or replace the filter so the car stays legal and passes its MOT.

Keeping your DPF healthy

Give a diesel a proper run now and then, keep up with servicing, and use the correct low-ash engine oil for your car, as the wrong oil can block the filter faster. Do not ignore other warning lights either, because engine faults often lead to DPF trouble.

If your DPF light is on, call us on 01922 682200 or book online and we will get to the bottom of it.